This invention relates to an electrically conductive composition of the alkali-developing type and photo-curable type useful for the formation of a fine electrode circuit on a front substrate of a plasma display panel (hereinafter abbreviated as a xe2x80x9cPDPxe2x80x9d) and a PDP provided with a front substrate having electrodes, particularly under layers (black layers) of bus electrodes, formed by the use of the above composition.
The PDP is a planar display for exhibiting pictures and pieces of information by utilizing the light emitted by plasma discharge. It is classified under the DC type and the AC type according to the structure of panel and the mode of driving. The principle of color display by the PDP consists in generating plasma discharge in cells (discharge spaces) between two opposed electrodes severally formed on a front glass substrate and a rear glass substrate separated by intervening ribs (barriers), and exciting the phosphor formed on the inner surface of the rear glass substrate with the ultraviolet light generated by the discharge of such a gas as He or Xe sealed in the cells thereby inducing generation of visible lights of three primary colors. The cells in the DC type PDP are divided by the component ribs of a lattice, whereas those in the AC type PDP are divided by the ribs which are arranged in parallel on the face of the substrate. In either case, the cells are divided by ribs. Now, a brief explanation is given below with reference to a drawing attached hereto.
FIG. 1 illustrates a typical construction of the planar discharge type PDP using a three-electrode structure for full color display. On the lower face of a front glass substrate 1, many pairs of display electrodes 2a, 2b each comprising a transparent electrode 3a or 3b intended for discharge and a bus electrode 4a or 4b intended for lowering the line resistance of the transparent electrode are formed with prescribed pitches. On the display electrodes 2a, 2b, a transparent dielectric layer 5 (low-melting glass) for accumulating electric charge is formed by printing and calcination. A protective layer (MgO) 6 is formed thereon by vacuum deposition. The protective layer 6 assumes the role of protecting the display electrodes and maintaining the state of discharge.
On a rear glass substrate 11, ribs (barriers) 12 shaped like stripes and adapted to partition discharge spaces and address electrodes (data electrodes) 13 severally disposed in the discharge spaces are formed with prescribed pitches. On the inner faces of discharge spaces, fluorescent films of the three colors, i.e. red (14a), blue (14b), and green (14c), are laid out regularly. In the full color display, the fluorescent films 14a, 14b, and 14c of the three primary colors of red, blue, and green mentioned above jointly form one picture element.
The PDP described above is called a xe2x80x9cplanar discharge systemxe2x80x9d because an AC pulse voltage is applied between the pair of display electrodes 2a, 2b to induce discharge between the electrodes on one and the same substrate. It has a construction such that the ultraviolet light generated by discharge excites the fluorescent films 14a, 14b, and 14c of the rear substrate 11 and the visible light consequently generated is seen through the transparent electrodes 3a, 3b of the front substrate 1 (reflection type).
The formation of the bus electrodes 4a, 4b mentioned above has been heretofore attained by a process which comprises forming three Cr/Cu/Cr layers by vacuum deposition or sputtering on the transparent electrode and then patterning the layers by the photolithographic technique. Since this process suffers copiousness of component steps and high cost of production, in recent years, a process which comprises screen printing a conductive paste such as of silver and then calcining the layer of the paste or, for the purpose of imparting to a pattern a line width of not more than 150 xcexcm, a process which comprises applying a photosensitive conductive paste to the transparent electrode, exposing the layer of the paste to light through a pattern mask, developing the exposed layer, and subsequently calcining the developed layer has come to take the place of the former process.
In the front substrate of PDP on which the bus.electrodes 4a and 4b are formed in this way, in recent years, the formation of the electrode of the black-white two-layer structure is performed to increase the contrast of images by the method comprising the steps of printing an electrically conductive black paste having a relatively poor conductivity as a lower layer (layer in contact with the transparent electrodes 3a and 3b) on a display side and then printing a white layer of silver paste having good conductivity thereon. In this case, a resin composition containing a silver powder and a heat-resistant black pigment is used as the electrically conductive black paste. In such an electrically conductive black paste, accordingly, it is necessary to adjust the amounts of silver powder and black pigment incorporated into the composition while balancing the conductivity and blackness thereof. However, under the actual circumstances it is hardly possible to obtain the, electrodes which may satisfy both the conductivity and blackness. That is to say, if a large amount of silver powder is incorporated into the composition in order to secure conductivity, an electrically conductive film of clear black cannot be obtained due to a silver color tone. Conversely, if a large amount of black pigment is incorporated into the composition for the purpose of attaining a clear black color, it is hardly possible to obtain an electrically conductive film having a desired conductivity.
Accordingly, the present invention has been made to solve the problems involved in such conventional technology. The main object of the present invention is to provide a photo-curable, electrically conductive composition which is capable of forming a conductive film satisfying both the sufficient conductivity and blackness after calcination, without deteriorating the excellent adhesiveness to a substrate, resolution, and calcination properties of the electrically conductive composition in the component steps of process, i.e. drying, exposure to light, development, and calcination.
Another object of the present invention is to provide a PDP provided with the front substrate on which a highly fine electrode circuit formed of such a photo-curable, electrically conductive composition, particularly bus electrodes each consisting of two layers of black and white in which the lower layer (black layer) electrode circuit satisfies both the sufficient conductivity between layers (electrical conduction between a transparent electrode and a white layer of the bus electrode) and the blackness, is formed.
To accomplish the objects mentioned above, according to the present invention, there is provided a photo-curable, electrically conductive composition. The first fundamental mode of the present invention is characterized by comprising (A) electrically conductive black fine particles having a specific surface area of more than 20 m2/g, (B) an organic binder, (C) a photopolymerizable monomer, and (D) a photopolymerization initiator. The second mode is characterized by further comprising (E) inorganic fine particles besides the above-mentioned ingredients. As the electrically conductive black fine particles (A) mentioned above, at least one kind of, fine particles selected from the group consisting of ruthenium oxides, ruthenium compounds, copper-chromium-based black composite oxides, and copper-iron-based black composite oxides are advantageously used. Such a photo-curable, electrically conductive composition of the present invention may be in the form of paste or in the form of a dry film produced in advance from the composition in the form of film.
In accordance with the present invention, there is further provided a PDP provided with a front substrate having an electrode circuit formed from a calcined product of such a photo-curable, electrically conductive composition.